Field of the Invention and Related Art Statement
The present invention relates to an apparatus for monitoring a bloodstream flowing through a blood vessel, and more particularly to an apparatus for measuring a distribution and a variation in time of an average velocity of bloodstream in a skin surface of a patient by means of the laser speckle method.
When laser light is made incident upon a living tissue such as a skin, the laser light is scattered by particles constituting the blood, and scattered light rays are interfered with each other to form random pattern, i.e. speckle pattern. This speckle pattern changes in time due to the movement of the blood cells in the blood vessel. Therefore there can be derived a noise-like signal representing the velocity of the bloodstream by measuring the variation in time of the intensity of light scattered at a certain point on the skin. This phenomenon was found by Dr. M. D. Stern et al around the year of 1975. By utilizing this phenomenon, the bloodstream can be measured with the aid of the frequency analysis of the speckle signal without damaging the skin. The study of this phenomenon has been rapidly developed and the apparatus for monitoring the bloodstream has been commercially available as the laser Doppler bloodstream meter.
In the known apparatus, the variation in time of the bloodstream at a point is detected with the aid of an optical fiber scope, or an abnormal condition is detected by comparing the detected data with that at a standard point. However, in the known laser Doppler bloodstream meter using the optical fiber, since an area of the detection point has a very small diameter such as several millimeters, the measured data fluctuates for respective measuring points. Therefore, it is not suitable to estimate the bloodstream in a rather large area. Further, since the signal obtained via the optical fiber scope is inherently noisy, there must be provided an integrating circuit or low pass filter for smoothing the noisy signal. When the time constant of these circuits is made large, the variation of the bloodstream can be displayed slowly, but the response to the rapid change in the bloodstream is decreased.